This invention relates to monitoring the dissipation of hydrogen atoms through a body comprised of material which is permeable to atomic hydrogen, but impervious to gases or liquids. This invention relates particularly to monitoring the accumulation of molecular hydrogen gas in an evacuated chamber resulting from the migration of hydrogen atoms through such a body.
A typical example of an application in which the present invention may be used is to monitor the presence and general magnitude of corrosion occurring at a surface of a ferrous metal body when exposed to a corrodent such as water as further discussed in U.S. Pat. No. 4,043,178 and U.S. Pat. No. 4,065,373.
It is well known that when ferrous metals are exposed to water and corrode that hydrogen atoms are liberated at a rate corresponding to the rate of corrosion activity as a result of the dissociation of water molecules into hydrogen atoms and hydroxyl groups. Some of these hydrogen atoms permeate and migrate through the ferrous metal body. When such atoms resurface they combine to form molecular hydrogen gas and then dissipate.
It is desirable to monitor the magnitude of corrosion occurring on the surfaces of such bodies to provide an indication of expected depletion of the corroding surface, as well as expected hydrogen blistering corrosion as discussed in U.S. Pat. No. 4,416,996, so that appropriate remedial action may be taken. Hydrogen blistering corrosion consists of hydrogen atoms diffusing through a ferrous metal body until an imperfection in the metal is reached. At this point the hydrogen atoms combine to form molecular hydrogen gas. As ferrous metals are impervious to molecular hydrogen this is a unidirectional process and results in the accumulation of pressures of several thousand psi. Upon attaining sufficient pressure the hydrogen gas causes the metal to deform forming a blister and eventually bursting.
Both the depletion of corroding surfaces and hydrogen blistering corrosion are harmful in that they result in reducing the cross section of the body thereby weakening it. Monitoring the magnitude of corrosion occurring at a surface permits preventative maintenance to be scheduled and conducted as required to maintain the structural integrity of the body.
Various systems have sought to exploit the solubility and dissipation of atomic hydrogen as a means of monitoring the corresponding magnitude of corrosion activity.
One such device uses a ferrous metal probe within which is formed a cavity. The probe is inserted through a wall of a body into a corrodent present therein. The resulting corrosion of the surface of the probe generates hydrogen atoms. Some of these hydrogen atoms dissipate through the wall of the probe and emerge within the cavity to form hydrogen gas.
Various measurement techniques have been used to determine the rate of production of hydrogen gas. One measurement technique used in conjunction with this type of probe measures the accumulation of hydrogen gas within the cavity over time using a pressure gauge attached to the probe remote from the corrodent.
Another measurement technique used with this type of probe continuously vents the hydrogen gas from the cavity into an ionization chamber and detector sensor whose output is measured by a scalar instrument to indicate rate of flow of the hydrogen gas.
Still another measurement technique used with this type of probe continuously vents the hydrogen gas from the cavity through a capillary port into a background liquid to form discrete uniform-dimension bubbles of hydrogen gas which are counted over time to provide the rate of flow of the hydrogen gas.
The probe type of corrosion monitoring device is of limited value because:
1. Installation of the probe requires penetrating a wall of the body to be monitored in order to place the probe within the corrodent. This compromises the integrity of the body's structure which is an important consideration in many applications as discussed in U.S. Pat. No. 4,065,373. Installation is also labour-intensive and can be a potential safety risk to installers in the event the corrodent comprises pressurized, heated, volatile or radioactive material. PA1 2. Direct monitoring of the magnitude of corrosion of the body can only be achieved if the probe is constructed of the same material as that of the body. A probe constructed of a different material may corrode in a different fashion and at a different rate. No two materials are identical. Even variations in the specifications within the same class of material during its manufacture is normal. Materials may also be non-homogeneous. These factors make an exact match of the body and probe materials very difficult. PA1 3. Heat affected or stressed areas, as well as areas displaying surface anomalies such as mill scale and metallurgical defects, can have a very significant effect on the surface's susceptibility to corrosion. This variance in corrosion susceptibility is a function of the materials and construction techniques used in making the body as well as the body's design. Probe type corrosion monitoring devices are unable to model or duplicate such variable susceptibility to corrosion. The indirect corrosion monitoring technique used in probe type corrosion monitoring devices cannot accurately monitor the magnitude of corrosion activity occurring in such bodies. PA1 4. The sensitivity of the pressure-measuring style of probe is a function of the rat(i of hydrogen gas accumulation within the cavity. This is dependant on the corroding surface area of the probe (where the migrating hydrogen atoms are generated), the interior surface area of the cavity (through which the migrating hydrogen atoms exit to form molecular hydrogen) and the volume of the cavity. Sensitivity of the pressure-measuring style of probe to the detection of hydrogen is limited in that a substantial period of time is required to accumulate a measurable volume of hydrogen gas within the fixed volume of the probe's cavity. U.S. Pat. No. 4,043,178 discusses that even in very active corrodents the pressure build-up in such probes requires up to 24 hours to reflect gas accumulations within the cavity of 85 psi. In field conditions this may take weeks. PA1 5. Pressure readings of the pressure-measuring style of probe will fluctuate with changes in ambient temperature inside or outside the body resulting in inaccurate indications of the magnitude of corrosion activity within the body. PA1 6. The probe style corrosion monitoring devices are relatively expensive to manufacture and install. In addition, the venting style of probe requires careful calibration and complicated installation and is relatively too delicate for use unattended in industrial field installations. PA1 1. Pressure readings from pressure-measuring hydrogen patch devices will fluctuate with changes in ambient temperature resulting in inaccurate indications of corrosion activity within the body; PA1 2. The sensitivity of a pressure-measuring hydrogen patch device is a function of the rate of hydrogen gas accumulation within its cavity. The sensitivity of hydrogen patch devices may generally be enhanced by maximizing the monitored surface area of the body within the cavity (through which the migrating hydrogen atoms exit to form molecular hydrogen) while minimizing the volume of the cavity. The sensitivity of a pressure-measuring hydrogen patch device is limited in that a substantial period of time is required for the accumulation of a measurable volume of hydrogen within the fixed volume of the cavity. This increases an operator's reaction time in commencing corrective action to limit corrosion-induced damage. PA1 3. Sensitivity of the electrochemical type of hydrogen patch is practically limited by the relatively fixed surface area that they monitor on the body. Such hydrogen patches are not easily adjusted to increase the surf ace area they monitor due to the nature of their construction. PA1 4. The flexibility of hydrogen patch devices to adapt to irregularly shaped body surfaces is also limited by the nature of their construction. PA1 5. The electrochemical patch cell is relatively expensive and complicated to manufacture and to install. PA1 (a) a chamber-defining member; PA1 (b) a steel body in which at least said selected area is impervious to the flow of gasses or liquids; PA1 (c) seal mans extending around the marginal perimeter of the chamber-defining member for sealably securing the chamber-defining member to the first surface so as to define with said selected area of the first surface of the body a sealed chamber which is impervious to the flow of gasses or liquids; PA1 (d) said chamber defining member being adapted to conform closely to the surface shape of said selected area of the first surface so as to lie in close proximity thereto to minimize the volume of the sealed chamber defined between the selected area of the first surface and the chamber-defining member; PA1 (e) means for connecting an evacuating means to the chamber defining member to permit substantial evacuation of the contents of the chamber to establish a partial vacuum therein; PA1 (f) valve means for isolating the chamber form the evacuation means after the partial vacuum has been established to maintain the partial vacuum in the chamber so that hydrogen atoms that are generated as a result of corrosion of the second surface of the body and which diffuse through the material of the body and which exit the first surface within the chamber and which combine to form hydrogen gas molecules, collect in the chamber thus resulting in a decay of the vacuum in the chamber; and PA1 (g) vacuum monitoring means for monitoring the decay of the vacuum in the chamber over time to give an indication of the rate of diffusion of hydrogen atoms through the material of the body and hence an indication of the rate of corrosion of said second surface. PA1 (a) providing a steel body having a first surface and an opposite second surface, wherein the body is impervious to the flow of gasses or liquids between said surfaces at least in a selected area of the body; PA1 (b) providing a chamber defining member capable of conforming closely to the surface shape of said selected area of the first surface so as to lie in close proximity thereto; PA1 (c) sealably engaging the chamber-defining member with the first surface of the body, using a seal means which extends around the marginal perimeter of the chamber-defining member, so as to define with said selected area of the first surface of the body a sealed chamber which is impervious to the flow of gasses or liquids; PA1 (d) substantially evacuating the contents of the chamber using evacuation means to establish a partial vacuum therein; PA1 (e) isolating the chamber from the evacuation means after the partial vacuum has been established to maintain the partial vacuum in the chamber so that hydrogen atoms that are generated as a result of corrosion of the second surface of the body and which diffuse through the material of the body and which exit the first surface within the chamber and which combine to form hydrogen gas molecules, collect in the chamber thus resulting in a decay of the vacuum in the chamber; and PA1 (f) monitoring the decay of the vacuum in the chamber over time to give an indication of the rate of diffusion of hydrogen atoms through the material of the body and hence an indication of the rate of corrosion of said second surface thereof.
Another device, known as a hydrogen patch, is discussed in U.S. Pat. No. 4,065,373. This style of monitoring device uses a cell mounted upon, and sealed to, the diffusion side of a body being penetrated by hydrogen atoms. The cell and the body define a cavity within which hydrogen gas forms and collects as migrating hydrogen atoms surface within the cavity.
Various measurement techniques have been used to monitor the rate of production of hydrogen within the cavity of such hydrogen patches. One such technique uses a pressure gauge to measure the accumulation of hydrogen gas over time. Another measurement technique, discussed in U.S. Pat. No. 4,065,373, electrochemically converts hydrogen atoms to ions. The rate of conversion of hydrogen atoms to ions is monitored electronically to provide a measurement of the magnitude of corrosion occurring inside the monitored body.
These hydrogen patch devices are of limited value because: